1. Field of the Invention
The present invention relates to non-invasive skin contact electrodes for measuring bioelectrical signals from the human body, particularly to electrodes applied to the scalp for the measurement of electroencephalogram (EEG) signals from the human brain.
2. Description of the Related Art
Conventional techniques for recording brain waves (electroencephalograms for EEGs) of a human subject require connecting electrodes on the scalp with a low impedance connection. This generally involves cleaning and abrading the scalp and applying a conducting gel or solution that makes the electrical contact between the scalp and the electrode. When performed manually, the procedure takes about 20 minutes for the nineteen electrodes usually used in clinical EEG examinations, and correspondingly longer when more electrodes are used for high resolution recordings.
Prior attempts to make the process of attaching electrodes to the scalp more efficient have met with limited success. A device by Johannson, Eralp and Ital (U.S. Pat. No. 4,683,892) is based on an electromechanical design which mechanized the manual scalp preparation procedure. Due to its bulk, weight and mechanical complexity, the device had limited utility.
In U.S. Pat. No. 4,709,702 to Sherwin, the electrodes contact the scalp with xe2x80x9ctulip probesxe2x80x9d having sharp points to xe2x80x9cpenetrate the dead skin layer.xe2x80x9d Such a sharp point tip is medically dangerous due to the possibility of infection and hurting the patient.
In U.S. Pat. No. 5,038,782 to Gevins, Durousseau and Libove, a dry electrode is described in which multiple metal conductive fingers protrude through the hair to the scalp. Because of the high impedance connection of the electrode tips with the scalp, the electrodes are excessively sensitive to artifacts resulting from head motion.
Taheri, Knight and Smith, xe2x80x9cA dry electrode for EEG recordingxe2x80x9d, Elect. and Cl. Neurophysiology 90 (1994), 376-383, describe a dry electrode in which one side of a 3 mm stainless steel disk is covered with a 200 nanometer thick silicon nitride coating that contacts the skin. The thin coating is fragile and easily damaged by handling, or by repeated or prolonged contact with the scalp. It is also difficult and therefore expensive to manufacture such a coating with the very low level of contaminants required for a capacitive electrode. The device attempts to compensate for poor quality electrode contact by using complex electronic circuits to sample multiple contacts to find the best one at any given instant.
Prutchi and Sagi-Dolev, xe2x80x9cNew Technologies for In-Flight Pasteless Bioelectrodes,xe2x80x9d Aviat. Space Environ. Med. 64 (1993) 552-556, describe a dry electrode using an aluminum plate with a hard aluminum oxide coating applied by a novel anodization process. The coating is subject to contamination by salt from the skin and also to scratching because it is very thin, rendering the device not practical.
In accordance with the present invention, a novel device called xe2x80x9cCeramic Single Plate Capacitor EEG Electrodexe2x80x9d is provided for measuring electrical potentials from the scalp or surface of the body.
The Ceramic Single Plate Capacitor EEG Electrode is a capacitive electrode. It is implemented as a capacitor with a single internal conducting plate, rather than the usual two plates of a conventional capacitor. The conducting plate is covered by a very high dielectric constant ceramic layer that acts as the insulating medium. Typically the relative dielectric constant is as great as 20,000. The conducting plate is separated from the patient""s scalp by the insulating ceramic layer. When placed in close proximity to the scalp, the patient""s brain in effect acts as the second plate of the capacitor. The xe2x80x9cceramicxe2x80x9d is normally a transition metal oxide composition which is generally formed by firing at a high temperature, i.e., sintered.
The main advantage of this electrode is that it measures brain waves or other physiological potentials without the need to cleanse and abrade the skin or apply conducting gels or liquids. The electrode can consequently be applied very quickly, and problems associated with the mess of the electrode gel and drying out of the electrode gel in long duration recordings are eliminated. Another advantage of the present electrode is that the possibility of shocking the patient through the electrode is virtually eliminated due to the high electrical isolation of the insulating ceramic layer. Additionally, electrolytic voltages generated in conventional wet electrodes by the virtual battery formed between the skin, the electrode gel, and the electrode, are eliminated. This, in principle, reduces artifacts due to head and body movements. The ceramic layer is relatively thick, 0.1 to 1.0 mm, hard and abrasion resistant. It has high capacitance (approximately 10-30 nanofarads). The ceramic layer, unlike thin film capacitors such as those that use silicon nitride, can withstand the mechanical abrasion that occurs in routine use without shorting out the capacitor. Because a robust contact is made, only a single contact point is necessary. Therefore, complex electronic circuits that sample multiple contacts in order to find a good one are unnecessary. Additionally, the electrode of the present invention may be manufactured inexpensively since the electrode is the equivalent of a parallel plate capacitor with one plate missing. It can be manufactured using conventional ceramic capacitor manufacturing facilities.
The ceramic electrode is preferably mounted on a miniature conventional printed circuit board with a preamplifier (using standard methods) and the entire assembly can be encapsulated and positioned on the head, or body, of the patient. The preamp buffers the high impedance signal from the electrode, drives the cable, and supplies some shielding of the electrode with its internal ground plane. Additional shielding is provided by a cap or electrode holder that keeps the electrode in contact with the patient.
The contact quality and electrical gain characteristics of the electrode-to-patient contact can be ascertained using a small injected signal on the patient ground contact. This provides a means of automatically checking the electrode hookup for problems. The idea is to apply a unit step of a few millivolts on the ground electrode of the patient. The RC network formed by the input resistor on the preamplifier and the capacitor electrode causes a decay in the voltage out of the preamplifier. If the contact to the patient is good, this capacity is high, in the 10 nanofarad range, and the circuit time constant is long. If the contact is poor, this capacity drops and is measured as a short time constant. This decay provides the information needed to compensate the electrode gain at low frequencies, and indicates if the capacity is less than expected because of poor contact quality. There is also a quick discharge phase that can be measured to determine the contact quality as well. The quick discharge phase can indicate the electrode has an area that is not in contact with the patient.
It is the objective of the present invention to provide an electrode to:
1. Record brain waves, or other physiological electrical potentials from the body, without the need to cleanse and abrade the skin or applying conducting gels or liquids;
2. Provide an electrode with high resistivity and capacitance;
3. Provide an electrode with a surface that is resistant to mechanical abrasion, to permit long duration recordings or repeated recordings;
4. Provide an electrode that is relatively simple and inexpensive to manufacture;
5. Provide an electrode that can be mounted on a conventional miniature printed circuit board, along with a preamplifier, using standard manufacturing methods;
6. Provide an electrode and amplifier assembly that can be encapsulated and positioned on a patient""s head, or body, in order to minimize interference from ambient electrical noise and to generate a low impedance output signal that is relatively insensitive to interference; and
7. Provide a means of automatically measuring the quality (impedance) of the contact between the electrode and the patient using a small injected signal on the patient ground contact.
It is a feature of the present invention to record brain waves, or other physiological electrical potentials, without the need to cleanse and abrade the skin conducting gels or liquids.
It is a further feature of the present invention to provide an electrode with a very high resistivity (greater than 100 Giga-ohm) and capacitance (approximately 30 nanofarads).
It is a further feature of the present invention to provide an electrode with a ceramic surface that is resistant to mechanical abrasion so that it may be used for long duration recordings, used multiple times, and cleaned and reused easily with conventional methods.
It is a further feature of the present invention to provide an electrode that is relatively simple and inexpensive to manufacture.
It is a further feature of the present invention to provide an electrode that can be mounted on a conventional miniature printed circuit board, along with a preamplifier, using standard manufacturing methods.
It is a further feature of the present invention to provide an electrode and amplifier assembly that can be encapsulated and positioned on a patient in order to provide immunity to ambient electrical noise and to generate a low impedance output signal that is relatively insensitive to interference.
It is a further feature of the present invention to provide a means of measuring the quality of the contact between the electrode and the patient using a small injected signal on the patient ground contact.
It is a further feature of the present invention to provide an electrode in which special electrode shapes are possible. For example, the ceramic can be shaped to have bumps or grooves to part the hair, be long and narrow or triangular to separate the hair during application, etc.